The reference index of this invention has specific application in the field of aircraft instrumentation and is specifically adapted to altitude indicating instruments or altimeters. Heretofore, attempts have been made to provide selectively positionable index or reference markers which are selectively positionable with respect to the face of the instrument dial for manual reference as to a selected or assigned altitude, or a restricted or limitative altitude which must be adhered to or strictly observed during the course of aircraft flight. The usual altimeters that are available and installed in aircraft are seldom provided with reference indexes as an element with original equipment structure. Accordingly, it is necessary to provide such reference index devices as auxiliary components in the many instances desired for facilitating maintenance of the assigned altitudes and aiding in observance of minimum altitude restrictions on instrument approaches. The known auxiliary devices available for attachment to altimeters are of a relatively expensive and cumbersome nature and are incapable of providing the desired distinctive indication that enhances the pilot's awareness of a specific assigned altitude or altitude limitation.
Another disadvantage of the known devices commercially available for this purpose, is that they are incapable of providing any selectivity as to two substantially distinct modes of operation normally encountered during flight. These two modes of operation are the maintenance of an assigned altitude on departure from an airfield and during the cruise aspects of the flight. There is also a period of time upon initial descent into an airport traffic control area wherein other assigned altitudes are given by approach controllers for effecting a safe expeditious movement of numerous aircraft through the control area. The second mode of operation desired for an altimeter reference index is associated with the instrument approach phase. Instrument approaches have either a decision height or minimum descent altitude (DH/MDA) associated with the respective approach. These altitudes are minimum restrictions that must be strictly observed for maintenance of safety and avoiding violation of aeronautical regulations.